1. Field of the Invention
The present invention relates to an image forming apparatus and to a unit removably installed in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copier or laser printer, it is preferred that image forming conditions be changed in accordance with the type of printing medium. There are various types of printing media, examples of which are plain paper, glossy paper, rough paper, thick paper, thin paper, OHT (overhead transparency) and the like. Preferably, the quality of the image formed on the printing medium is stabilized by setting image forming conditions in accordance with these types. For example, the size and type (also referred to as “type of paper” hereafter) of the printing medium is set by a user at a control panel, or the like, provided on the main body of the image forming apparatus, and fixing process conditions (e.g., fixing temperature and conveyance speed of the printing medium that passes by the fixing unit) are controlled in accordance with these settings.
It has been proposed to provide a printing-medium determination sensor within an image forming apparatus for the purpose of eliminating the task of having the user input the type of printing medium (see the specification of Japanese Patent Laid-Open No. 2002-182518). In accordance with this proposal, the type of printing medium is determined by sensing an image on the surface of the printing medium using a CMOS sensor, and the image forming conditions (e.g., developing conditions and transfer conditions or fixing conditions) are controlled so as to be changed.
Furthermore, an apparatus has been proposed in which a light source is provided at a position opposing a sensor which, by detecting transmitted light from the printing medium, determines the type of printing medium and the thickness of the printing medium (see the specification of Japanese Patent Laid-Open No. 2001-139189).
A problem with the techniques described above is that if paper dust or toner, or the like, attaches itself to the surface of the LED or sensor due to repetition of image formation, type and thickness will be determined erroneously. In order to solve this problem, a method of mitigating the effects of adhered paper dust and toner, etc., by correcting (or calibrating) the output of a reflective-type optical sensor using a corrective plate has been considered.
However, in a case where an image forming apparatus is used over a prolonged period of time or a case where a large quantity of printing (image formation) is performed, it is highly likely that paper dust or toner will affix itself to the surface of the LED or sensor in a reflective-type optical sensor to such an extent that the contamination will exceed a fixed level. In other words, it may be assumed that determination accuracy will decline even if the sensor output is corrected. For example, in the case of an apparatus in which the image forming speed is high (the number of sheets on which images are formed per unit time is large), it is highly likely that a large amount of image formation will be performed over a long period of time. Moreover, a large number of sheets of the printing medium are conveyed in a high-speed apparatus. As a result, it is highly likely that paper dust will attach itself to the surface of the LED or sensor.
Further, consideration has been given to adopting an arrangement in which paper dust or toner that has attached itself to the surface of a reflective-type optical sensor is removed (cleaned off) by the user. In this case, however, labor to remove the paper dust or toner is required of the user. Further, it is necessary to adopt an arrangement in which the user is capable of accessing the reflective-type optical sensor. There is the danger that this will lead to an increase in the size of the apparatus.